In an image forming apparatus such as an electrophotographic apparatus or electrostatic recording apparatus, an electrostatic latent image formed on a photosensitive material is developed by a toner. Then, after the formed toner image is transferred to a transfer medium such as paper as required, the toner image is fixed by various methods such as heating, pressing and solvent-vapor treatment.
As the toner used in such an image forming apparatus as a developer, a toner produced by a pulverizing process, wherein a colorant, a charge control agent, a parting agent and the like are melted and mixed into a thermoplastic resin, which becomes a binder resin component, and dispersed evenly to form a composition, which is then pulverized and classified to obtain colored particles; a toner produced by suspension polymerization, wherein a colorant, a charge control agent, a parting agent and the like are dissolved or dispersed in a polymerizable monomer, which is a material for a binder resin, the monomer is suspended in an aqueous dispersing medium containing a dispersion stabilizer, heated to a predetermined temperature to initiate polymerization, filtered, washed, dehydrated and dried to obtain colored particles; or a toner produced by emulsion polymerization, wherein the particles of a binder resin containing polar groups are combined with particles containing a colorant and charge control agent are filtered, washed, dehydrated and dried to obtain colored particles; are used.
The fixing methods used in the image forming apparatus include pressing roller fixation, heating roller fixation, oven heating fixation, light radiation (flashing) fixation and solvent fixation. Among these, the fixing method using heating rollers wherein toner images on a transfer medium such as paper are passed between heating rollers is preferably used in view of the image quality or thermal efficiency. Although electric power is used for heating the rollers in the fixing method using heating rollers, the lowering of the fixing temperature is requested from the point of view of energy saving. From the aspect of toner design, this request is responded by lowering the melt viscosity of the binder resin.
To lower the melt viscosity of the binder resin obtained by ordinary radical polymerization, the molecular weight is decreased by the adjustment of the quantity of the initiator and the monomer ratio, or by the addition of a chain transfer agent, but the glass transition temperature of the binder resin is also lowered due to the occurrence of oligomers, resulting in a problem that the shelf stability is lowered.
In polymerization using aromatic vinyl compounds as the monomers, since the stop reaction occurs mainly by the two-molecule stop of the styrene radicals, a coupling reaction occurs. Therefore, when the molecular weight is measured, the molecular-weight distribution has a tailing in the high-molecular-weight side. The formation of high-molecular-weight bodies controlling the melt characteristics of the binder resin was not preferred especially for the resin for the color toner. Therefore, the lowering of the molecular weight of the binder resin is being devised.
As a method for controlling the occurrence of oligomers while reducing the molecular weight of the resin, living radical polymerization is being studied. Japanese Patent Application Laid-Open No. 11-315106 proposes a method for providing a polymer or a block polymer of a narrow molecular-weight distribution by polymerizing a polymerizable monomer using a radical initiator and a transition metal complex formed by the coordination of a specific ligand to the transition metal in an emulsion polymerization system.
However, according to examples, although polymers of a narrow molecular-weight distribution are surely formed, the polymerization conversion is as low as 60 to 90% even after 5 to 6 hours have elapsed, and a large quantity of monomers remain after polymerization. When this method is applied to the toner, the odor after fixation raises a problem. Especially when the toner is adopted as a color toner, the coloring properties demanded to the toner is impaired because transition metals are contained.
In the color toner, since the sharp melt properties of the resin is demanded, only a polyester-based copolymer has been used as the binder resin in the toner produced by the pulverizing method. Japanese Patent Application Laid-Open No. 2001-42571 discloses a toner that excels in fixing and charge properties using a polyester-based resin, a parting agent and a styrene acrylate-based resin containing a quaternary base. However, the pulverizing method that compounds large quantities of parting agent and low-molecular-weight wax, since the wax is unevenly distributed on the surfaces of the toner, the anti-filming and charge properties are affected causing problems.